How do I autocrop a UIImage?

Question

I have a UIImage which contains a shape; the rest is transparent. I\'d like to get another UIImage by cropping out as much of the transparent part as possible, still retain

Answer 1

Swift 3 (it's not a UIImage extension, I needed it in another class) it may save time for someone:

class EditImage {

static func cropRect(_ image: UIImage) -> CGRect {
    let cgImage = image.cgImage
    let context = createARGBBitmapContextFromImage(inImage: cgImage!)
    if context == nil {
        return CGRect.zero
    }

    let height = CGFloat(cgImage!.height)
    let width = CGFloat(cgImage!.width)

    let rect = CGRect(x: 0, y: 0, width: width, height: height)
    context?.draw(cgImage!, in: rect)

    //let data = UnsafePointer(CGBitmapContextGetData(context))
    let data = context?.data?.assumingMemoryBound(to: UInt8.self)

    if data == nil {
        return CGRect.zero
    }

    var lowX = width
    var lowY = height
    var highX: CGFloat = 0
    var highY: CGFloat = 0

    let heightInt = Int(height)
    let widthInt = Int(width)
    //Filter through data and look for non-transparent pixels.
    for y in (0 ..< heightInt) {
        let y = CGFloat(y)
        for x in (0 ..< widthInt) {
            let x = CGFloat(x)
            let pixelIndex = (width * y + x) * 4 /* 4 for A, R, G, B */

            if data?[Int(pixelIndex)] != 0 { //Alpha value is not zero pixel is not transparent.
                if (x < lowX) {
                    lowX = x
                }
                if (x > highX) {
                    highX = x
                }
                if (y < lowY) {
                    lowY = y
                }
                if (y > highY) {
                    highY = y
                }
            }
        }
    }


    return CGRect(x: lowX, y: lowY, width: highX - lowY, height: highY - lowY)
}

static func createARGBBitmapContextFromImage(inImage: CGImage) -> CGContext? {

    let width = inImage.width
    let height = inImage.height

    let bitmapBytesPerRow = width * 4
    let bitmapByteCount = bitmapBytesPerRow * height

    let colorSpace = CGColorSpaceCreateDeviceRGB()
    if colorSpace == nil {
        return nil
    }

    let bitmapData = malloc(bitmapByteCount)
    if bitmapData == nil {
        return nil
    }

    let context = CGContext (data: bitmapData,
        width: width,
        height: height,
        bitsPerComponent: 8,      // bits per component
        bytesPerRow: bitmapBytesPerRow,
        space: colorSpace,
    bitmapInfo: CGImageAlphaInfo.premultipliedFirst.rawValue)

    return context
}
}